Tabular silver halide grains are crystals possessing two major faces that are substantially parallel. The average diameter of said faces is at least three times the distance separating them (the thickness), this is generally described in the art as an aspect ratio of at least 3.
Silver halide photographic emulsions containing a high proportion of tabular grains have advantages of good developability, improved covering power and increased useful adsorption of sensitizing dye per weight of silver due to their high surface area-to-volume ratio. The use of such emulsions in photographic elements is disclosed in U.S. Pat. Nos. 4,425,425, 4,425,426, 4,433,048, 4,435,499, 4,439,520, and other related patents.
The use of automatic processors for the rapid processing of light-sensitive silver halide elements including tabular silver halide grains, in particular light-sensitive silver halide elements for radiographic use, is known. Such elements generally include a support (usually provided with a very thin subbing layer) having coated on at least one side a silver halide gelatin emulsion layer coated in turn with a gelatin protective layer. These elements are transported through the machine processing units (developing, fixing, washing and drying) by means of opposed or staggered rollers (as described, for example, in U.S. Pat. No. 3,025,779) which also have the function of sqeezing liquid from the film prior to drying. In order to reduce the time taken by the element to pass through the processing machine to 0.5 to 2 minutes, as particularly required in rapid processing of radiographic elements, the processing is performed at relatively high temperatures, usually higher than 30.degree. C., preferably between 35.degree. and 45.degree. C., such as 38.degree. C., and the gelatin content of the silver halide emulsions is considerably reduced as compared with that of emulsions for manual processing.
Under such conditions, even with the changes in the emulsions, the physical and photographic properties of the elements processed in an automatic processor tend to be worse. With high temperatures and in presence of such low gelatin content, for instance, the intrinsic sensitivity to pressure of the silver halide grains gets higher and the elements processed in the automatic processor show marks caused by the pressure of the transporting rollers. Such pressure marks look like higher density regions and reduce the image faithfulness.
In order to prevent pressure marking, various methods have been described in the art. To this purpose, U.S. Pat. No. 2,960,404 describes the use in the photographic elements of glycerine, ethylene glycol and the like, Japanese Pat. No. 5316/1972 describes the use of 1,4-cyclohexane dimethanol and the like, and Japanese Pat. No. 4939/1978 describes the use of trimethylol propane. Another possible method of preventing pressure marking is by increasing the degree of hardening of the gelatin layers, in particular of the external protective layers. As another method, photographic elements are known wherein an intermediate gelatin layer is interposed between the support and the emulsion layer. For example, U.S. Pat. No. 3,637,389 describes a rapid processing photographic element wherein gradation, density and sensitivity are improved by applying such an intermediate gelatin layer between the support and the emulsion layer.
However, known methods of preventing pressure marking when used in photographic elements including tabular silver halide grains have proved less effective. Accordingly, the problem still remains of preventing pressure marking in photographic elements including light-sensitive tabular silver halide emulsions.
U.S. Pat. No. 4,414,304 describes forehardened photographic elements, particularly radiographic elements, including at least one hydrophilic colloid emulsion layer containing tabular silver halide grains. The element require no additional hardening on development and give images of high covering power. Among gelatin hardeners, bis(vinylsulfonylmethyl) ether, mucochloric acid and formaldehyde are described.
Japanese patent application no. J5 9105-636 describes photographic elements comprising at least one silver halide emulsion layer containing tabular silver halide grains, the binder of at least one of the hydrophilic colloidal layers being gelatin which has jelly strength of at least 250 g. Wet coat strength of said elements is improved without reducing covering power.
DE patent application no. 3,433,893 describes photographic elements containing tabular silver halide grains and a polymeric hardener. The elements have increased resistance to scratching during wet processing and good covering power.
Japanese patent application no. J6 2249-140 describes photographic elements comprising at least one silver halide emulsion layer containing tabular silver halide grains and halogen substituted s-triazine type hardeners. The elements are suitable for rapid processing and have improved pressure resistance.
U.S. Pat. No. 4,847,189 describes a photographic element comprising at least one silver halide emulsion layer containing tabular silver halide grains, the melting time and the gelatin amount of the element being such as to render the element suitable for rapid processing and improve the pressure desensitization resistance.